The establishment of immunotherapies based on thymus--derived lymphocytes (T cells) as a treatment modality for cancer and other diseases in humans is an area of considerable research interest (Oethgen, H. F. et al. (1991) in Biologic Therapy of Cancer: eds.: DeVita, V. T. Jr., Hellman, S., Rosenberg, S. A. J. B. Lippincott, p. 87). A major hindrance to the development of effective T cell-based immunotherapies is that antigen presentation on the surface of cells is often inadequate to elicit a T cell response to the antigen. Thus, a major aim of researchers in fields such as cancer biology, virology and immunology is the development of methods which enhance the presentation of antigens to T cells. In order to better understand the present invention, a brief review of how T cells recognize, or fail to recognize, antigens is presented below (see also Restifo, N. P Biologic Therapy of Cancer Updates 2: 1-10 (1992); Yewdell, J. W. Adv. in Immunology 52: 1-123 (1992)).
Unlike B cells which can recognize antigens not presented in the context of other molecules, T cells can only recognize antigens in the context of a major histocompatibility complex (MHC) on the surface of a target cell. In particular, two types of MHC molecules exist and each type, noncovalently linked with antigenic peptides, constitutes a ligand for different subsets of T cells. More specifically, class I MHC/peptide complexes are recognized by CD8.sup.+ T cells while class II MHC/peptide complexes are recognized by CD4.sup.+ T cells. Of interest to researchers involved in the development of T cell based immunotherapies, CD8.sup.+ T cells, sometimes termed cytotoxic T lymphocytes or CTLs, have been demonstrated to be capable of directly killing target cells presenting a class I/peptide complex on their cell surface and of secreting cytokines which may signal for the destruction of these target cells. These properties of CD8.sup.+ T cells have stimulated numerous investigators to focus on the study of the processes leading to the formation of class I/peptide complexes within target cells and the subsequent presentation of these complexes on the surface of the target cells in order to better understand the molecular apparati involved in the presentation of peptides to CD8.sup.+ T cells. To date, although the processes involved in the cleavage and transport of peptides that are bound by class I MHC molecules are only now being characterized, some details are known.
In brief, the generation of antigenic peptides for class I molecules from cytosolic proteins (Tevethia, S. S., et al. Virology 107: 13-23 (1980); Bennink, J. R., et al. Nature 296: 75-76 (1982); Yewdell, J. W., et al. Proc. Natl. Acad. Sci. USA 82: 1785-1789 (1985); Yewdell, J. W., et al. Science 239: 637-640 (1988); Townsend, A. R. M., et al. Cell 39, 13-25 (1984)) is achieved by unknown cytosolic proteases. Once formed in the cytosol, these peptides are then delivered to the endoplasmic reticulum (ER) via a process which requires the presence of two MHC encoded gene products termed TAP 1 and TAP 2 (Deverson, E., et al. Nature 348: 738-741 (1990); Trowsdale, J., et al. 348: 741-744 (1990); Spies, T., et al. Nature 348: 744-747 (1990); Monaco, J. J., et al. Science 250: 1723-1726 (1990). In the ER, the peptides associate noncovalently with class I MHC molecules to form a class I MHC/peptide complex which is then transported to the cell surface. The class I/peptide complex presented on the cell surface is now capable of serving as a ligand for cell surface receptors on CD8.sup.+ T cells and hence, of eliciting a T cell response against the presented peptide. Due to the complexity of the processing pathways which ultimately results in antigen presentation to CD8.sup.+ T cells, deficiencies in expression of any of the components of the antigen processing pathways outlined above might be expected to result in reduced presentation of antigen to CTLs.
Recent studies by both Eisenlohr et al. (Cell 71: 963-972 (1992)) and Anderson et al (J. Exp. Med. 174: 489-492 (1991)) have demonstrated that although presentation of antigens to CTLs is dramatically reduced in a cell line having deletions in the genes encoding TAP 1 and TAP 2 relative to that observed in control cells, efficient antigen presentation in a TAP-deficient cell line could be achieved via transfection of these cells with "minigenes" in which the antigenic peptide was placed immediately carboxy-terminal to an ER signal sequence. Such signal sequences are generally found at the NH.sub.2 -terminus of proteins and their function is to target such proteins to the ER membrane. It should be noted however that the enhancing effect of the ER signal sequence on antigen presentation observed in these studies was not noted in control cells and was therefore, only observed in in vitro transfection or infection of a TAP-deficient cell line. However, evidence supporting the idea that the presentation of antigens processed from the cytosol might be limiting in vivo was recently provided by the observation by other investigators that TAP 1 and TAP 2 expression is enhanced following exposure of cells to gamma--interferon (Trowsdale, J., et al. Cell, 348: 741-744 (1990). This result suggested that TAP-mediated peptide delivery can be limiting in vivo as well as in vitro and that therefore, methods which could enhance the transport of peptides in vivo, or bypass transport activity entirely, might result in enhanced presentation of peptides to T cells.